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Understand Extracting Substances with Electrolysis

Worksheet Overview

QUESTION 1 of 10

Extracting aluminium from aluminium oxide

Aluminium is an incredibly useful metal, because it is light and strong. In nature, we find it as aluminium oxide in rocks, and for a long time it was hard to extract aluminium metal from the oxide. Electrolysis of molten aluminium oxide is the way we can do this.

We cannot do electrolysis on solids, and aluminium oxide does not dissolve in water. This means that we have to melt aluminium oxide for electrolysis to happen. The melting point of pure aluminium oxide is very high (over 2000 ºC), but adding another mineral, called cryolite, reduces the melting temperature to about 1000 ºC. This is helpful, because it reduces the cost of the process.

Graphite electrodes are used, made of pure carbon. These are placed in the molten aluminium oxide/cryolite mixture. Then electrolysis starts.

Aluminium ions are attracted to the cathode, where they form aluminium atoms. The ionic equation is Al3+ + 3 e- → Al.

The molten metal sinks to the bottom of the vessel, where it falls out and is collected.

Oxide anions are attracted to the anode, where they form oxygen. The ionic equation is 2 O2- → O2 + 4 e-.

The oxygen reacts with the carbon electrodes, making carbon dioxide. This gradually destroys the electrodes, so they have to be regularly replaced.

Electrolysis of aluminium oxide is expensive, for several reasons.The main ones are

the cost of heating the aluminium oxide

the cost of electricity for the electrolysis

Because of this, aluminium is expensive, which is why it is so important to recycle aluminium.

Extracting chlorine from sodium chloride

Chlorine is also a very useful element. It is used for cleaning, as a disinfectant and in bleach. Chlorine is added to water (especially in swimming pools) to kill bacteria. Some plastics (especially PVC) also need chlorine as an ingredient. Our main source of chlorine is sodium chloride. We use electrolysis to get chlorine out of sodium chloride.

Our first job is to find sodium chloride. Although there is some sodium chloride in sea water, it is better to find layers of sodium chloride buried underground in mines like this.

The easiest way to bring sodium chloride to the surface is to pump fresh water into the mine. Sodium chloride will dissolve into the water, so salty water will be pumped back to the surface. This gives us a solution which we can use for electrolysis.

The ions in the salt water electrolyte are Na+, Cl-, H+ and OH-. Sodium and hydrogen ions will be attracted to the cathode, and chloride and hydroxide will be attracted to the anode.

At the cathode, hydrogen ions are turned into hydrogen atoms; 2 H+ + 2e- → H2. Sodium ions stay in the solution as ions.

At the anode, chloride ions are turned into chlorine atoms; 2 Cl- → Cl2 + 2e-. Hydroxide ions stay in the solution. 

Electrolysis of sodium chloride is much cheaper than electrolysis of aluminium oxide. We can make the liquid electrolyte by dissolving salt in water, which doesn't need energy- unlike melting aluminium oxide.

Why do we add cryolite to the aluminium oxide? 

What happens at the cathode? Tick all the statements that apply. 

oxide is oxidised

aluminium is oxidised

aluminium is reduced

Al3+ + 3 e- → Al

Al → Al3+ + 3 e-

2 O2- → O2 + 4 e-

Fill in the gaps in this paragraph about aluminium. Use these words:

dissolve

electrolyte

electrolysis

energy

light

melt

metal

reactive

oxide is oxidised

aluminium is oxidised

aluminium is reduced

Al3+ + 3 e- → Al

Al → Al3+ + 3 e-

2 O2- → O2 + 4 e-

What happens at the cathode? Tick all the statements that apply. 

oxide is oxidised

aluminium is oxidised

aluminium is reduced

Al3+ + 3 e- → Al

Al → Al3+ + 3 e-

2 O2- → O2 + 4 e-

Why do the electrodes need to be replaced frequently?

The break off at high temperatures.

They slowly melt at high temperatures.

They react with oxygen to make carbon dioxide.

Sodium chloride is our main source of chlorine; where do we find sodium chloride?

In sea water

In river water

In mines underground

Where and how is chlorine formed? 

In sea water

In river water

In mines underground

Hydrogen and sodium are both attracted to the cathode. What happens to each of them?

In sea water

In river water

In mines underground

What is the half-equation for the cathode? 

H+ + e- → H

H2 → 2 H+ + 2 e-

2 H+ + 2 e- → H2

Why is it cheaper to produce chlorine than aluminium?

Chlorine is less polluting than sodium

We don't need to melt sodium chloride, unlike aluminium oxide

Aluminium oxide is scarcer than sodium chloride

  • Question 1

Why do we add cryolite to the aluminium oxide? 

CORRECT ANSWER
EDDIE SAYS
Heating large amounts of rock to 2000 ºC is expensive, so anything we can do to reduce this temperature is worthwhile. Mixtures tend to have lower melting temperatures than pure substances; it's the same reason we put salt on roads to stop water turning into ice.
  • Question 2

What happens at the cathode? Tick all the statements that apply. 

CORRECT ANSWER
aluminium is reduced
Al3+ + 3 e- → Al
EDDIE SAYS
The basic idea is that cations go to the cathode, as for any other electrolysis. They then get converted back into atoms by adding electrons. Gaining electrons is called reduction (remember OILRIG).
  • Question 3

Fill in the gaps in this paragraph about aluminium. Use these words:

dissolve

electrolyte

electrolysis

energy

light

melt

metal

reactive

CORRECT ANSWER
EDDIE SAYS
This all comes back to the reactivity series, really- aluminium is quite high up, so it readily goes into compounds and is hard to extract again. Although it is hard and expensive to make metallic aluminium, it is so useful that it\'s worth doing.
  • Question 4

What happens at the cathode? Tick all the statements that apply. 

CORRECT ANSWER
aluminium is reduced
Al3+ + 3 e- → Al
EDDIE SAYS
The basic idea is that cations go to the cathode, as for any other electrolysis. They then get converted back into atoms by adding electrons. Gaining electrons is called reduction (remember OILRIG).
  • Question 5

Why do the electrodes need to be replaced frequently?

CORRECT ANSWER
They react with oxygen to make carbon dioxide.
EDDIE SAYS
Graphite has a high melting temperature. The big problem is that electrolysis of aluminium oxide releases lots of oxygen, which then reacts with the carbon in the graphite electrodes.
  • Question 6

Sodium chloride is our main source of chlorine; where do we find sodium chloride?

CORRECT ANSWER
In sea water
In mines underground
EDDIE SAYS
We can get sodium chloride by evaporating water out of seawater. This can create enough salt for food, but not the huge amounts we need to make industrial quantities of chlorine, we use layers of salt from underground mines.
  • Question 7

Where and how is chlorine formed? 

CORRECT ANSWER
EDDIE SAYS
Chlorine is a non-metal, so it forms anions which go to the anode. When they get there, they lose electrons (oxidation). Chlorine then forms diatomic molecules, which become a gas.
  • Question 8

Hydrogen and sodium are both attracted to the cathode. What happens to each of them?

CORRECT ANSWER
EDDIE SAYS
Reduction happens at the cathode; the cathode has excess electrons, which go to the ions. Since sodium is very reactive, the electrons are transferred to the hydrogen (turning hydrogen ions into hydrogen gas), and the sodium stays as ions in solution.
  • Question 9

What is the half-equation for the cathode? 

CORRECT ANSWER
2 H+ + 2 e- → H2
EDDIE SAYS
Hydrogen undergoes reduction, so it gains an electron. The whole equation needs to be doubled, so that we can make H2 molecules.
  • Question 10

Why is it cheaper to produce chlorine than aluminium?

CORRECT ANSWER
We don't need to melt sodium chloride, unlike aluminium oxide
EDDIE SAYS
The main cost in doing electrolysis on aluminium oxide is the need to heat the oxide so much to melt it. We don't have that problem with sodium chloride, because it dissolves in water, which makes production much cheaper.
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